This invention relates to methods and apparatus for dynamically controlling the temperature of an ink-jet printhead.
An ink-jet printer includes at least one print cartridge that contains ink within a reservoir. The reservoir is connected to a printhead that is mounted to the body of the cartridge. The printhead is controlled for ejecting minute drops of ink from the printhead to a sheet of print medium, such as paper, that is advanced through the printer.
Many ink-jet printers include a carriage for holding the print cartridge. The carriage is scanned across the width of the paper, and the ejection of the drops onto the paper is controlled to form a swath of an image with each scan. Between carriage scans, the paper is advanced so that the next swath of the image may be printed. Sometimes, more than one swath is printed before the paper is advanced.
The printheads of modern ink-jet printers are capable of high-speed, high-resolution printing. Heat from the printhead firing resistors is transferred to the other printhead components. Also, the data carrying the information to be printed may be quite dense in instances where, for example, high-resolution color images are to be printed. As a consequence, printing tasks specifying high print density (that is, requiring relatively high numbers of ink drops over a unit area) can cause the operating temperature of the printhead to approach the maximum operating temperature of the printhead.
The maximum operating temperature of the printhead is established to ensure that the printhead is not operated at a temperature level that might cause the printhead to fail or otherwise diminish print quality. In this regard, it is possible to operate some printheads at a temperature level above the state transition temperature of some of the printhead components. Such operation would lead to complete failure of the printhead.
One past approach to controlling the printhead temperature involved the steps of examining the print density of a particular print job using thermal potential modeling. This modeling is reflects an empirically derived relationship between various print densities and associated thermal characteristics. The modeling reveals how hot the printhead may become for given print densities. In instances where the thermal potential model shows that the printhead maximum operating temperature would be exceeded, steps were taken to operate the printhead at a lower temperature. Such steps included slowing the printing operation by using more scans to print one swath, or by introducing cooling delays in the printhead operation upon the completion of each scan. Examining print data for thermal potential modeling is expensive in terms of system memory and processing.
Many past approaches to sensing printhead temperature feature sensors that can not be read with sufficient frequency to enable true dynamic temperature control of the printhead. As a result, the printheads were, typically, conservatively operated at temperatures significantly below their maximum operating temperature. Such conservative operation thus introduced unneeded delay in printing (such as unnecessarily long post-scan cooling delays) to ensure that the printhead did not exceed its maximum operating temperature.
The present invention is generally directed to a dynamic approach to controlling printhead temperature. The control is available during individual scans of the printhead across the media. Thermal potential modeling is not required in the method undertaken in the present invention. As a result, the processing and memory costs associated with thermal potential modeling is avoided.
The present invention incorporates high-frequency sampling of the printhead""s temperature in combination with a technique for estimating whether the printhead will exceed its maximum operating temperature during a scan across the media. If the estimation shows that the maximum operating temperature will be exceeded, the printing operation is halted (not aborted) at a convenient location in the scan. The unprinted data of that scan is preserved in memory, and the printhead is allowed to cool. The carriage (hence the printhead) is returned to the beginning of the scan during which the printing was halted (the paper is not advanced in the interim), the scan is restarted, and the printhead is controlled to recommence firing ink drops at the location in the scan where the firing had been stopped.
The firmware module that is responsible for delivering the firing signals to the printhead controls the stopping and restarting of the printhead firing. The printhead firing is stopped at a convenient location in the stream of print data, such as between byte or word boundaries in the data. As a result, the printing operation can be recommenced at the precise location it was stopped, thereby avoiding print defects that might otherwise occur if the mid-scan stopping and starting of the printhead were not so controlled.
As another aspect of this invention, the technique for estimating whether the printhead maximum operating temperature may be exceeded takes into account factors (such as the distance to the end of the scan) that, in combination with high-frequency temperature sensing, enables the operation of the printhead at a temperature very close to its maximum operating temperature, without exceeding that temperature. Other advantages and features of the present invention will become clear upon review of the following portions of this specification and the drawings.